Antiaromaticity–Aromaticity Interplay in Fused Benzenoid Systems Using Molecular Electrostatic Potential Topology

Abstract

The phenomenon of antiaromaticity–aromaticity interplay in aromatic–antiaromatic (A–aA)-fused systems is studied using molecular electrostatic potential (MESP) analysis, which clearly brings out the electron-rich π-regions of molecular systems. Benzene, naphthalene, phenanthrene, and pyrene are the aromatic units and cyclobutadiene and pentalene are the antiaromatic units considered to construct the A–aA-fused systems. The fused system is seen to reduce the antiaromaticity by adopting a configuration containing the least number of localized bonds over antiaromatic moieties. This is clearly observed in 25 isomers of a fused system composed of three naphthalene and two cyclobutadiene units. Denoting the number of π-bonds in the cyclobutadiene rings by the notation (n, n′), the systems belonging to the class (0, 0) and (2, 2) turn out to be the most and least stable configurations, respectively. The stability of the fused system depends on the empty π-character of the antiaromatic ring, hence naphthalene and benzene prefer to fuse with cyclobutadiene in a linear and angular fashion, respectively. Generally, a configuration with the maximum number of ‘empty’ rings (0, 0, 0, ...) is considered to be the most stable for the given A–aA system. The stability and aromatic/antiaromatic character of A–aA-fused systems with pentalene is also interpreted in a similar way. MESP topology, clearly bringing out the distribution of double bonds in the fused systems, leads to a simple interpretation of the aromatic/antiaromatic character of them. Also, it leads to powerful predictions on stable macrocyclic A–aA systems.

Unveiling the Multiradical Character of the Biphenylene Network and Its Anisotropic Charge Transport

“…Such states have not yet been reported for the 2D material, but there is some indication of their existence in the analogous 0D and 1D systems. Specifically, the non-bonding character of four-membered rings in molecular BP was demonstrated via molecular electrostatic potential topology calculations . Additionally, a recent study predicted the appearance of an antiferromagnetically aligned multiradical state in 1D BP, although this magnetic solution could not be detected in the on-surface synthesized material, potentially due to electron doping by the metallic surface.…”

“…As can be seen in Figure b,c, BPN displays a preference for electron transport through the CBD-like unit ( y -direction) rather than through the single C–C bonds connecting the hexagonal rings ( x -direction), for both electronic solutions. This goes in line with prior theoretical results showing the metallic character of hexagonal rings when connected via CBD-like square units and suggests that BPN may behave as an array of weakly coupled 1D BP channels.…”

BenzAI: A Program to Design Benzenoids with Defined Properties Using Constraint Programming

“…They are also studied in detail in interstellar chemistry because they are suspected to be present in interstellar clouds and are believed to act as catalysts for chemical reactions taking place in space . The aromatic character of benzenoids can be studied using the Clar sextet rule for ground states. − For large benzenoids, the shape of edges has a large influence on their properties. , They are also a challenge for synthetic chemists, which leads to the synthesis of remarkable molecules on surfaces . All these points can be addressed with BenzAI.…”

Competition Between Sextet Aromaticity and Cyclobutadiene Antiaromaticity within the Same Molecule: Location is Important

“…The results in Figure 4 agree with the higher level quantum results of Maksić and coworkers [15] who found that angular phenylene isomers are slightly more stable than their linear counterparts and explained this as being the result of decreased antiaromatic character of the planar four-membered rings. More recently Suresh and coworkers [42] also found that angular [3]phenylene was slightly more stable than its linear [3]phenylene isomer. This was rationalized as being the result of adopting a configuration containing the least number of localized bonds over the antiaromatic cyclobutadiene moieties which is equivalent to being the result of decreased antiaromatic character of the planar four-membered rings.…”